This invention relates to a semiconductor device having a ferroelectric capacitor and its manufacturing method.
For years, there are known non-volatile ferroelectric memory devices for non-volatile storage of data by using spontaneous polarization of a ferroelectric capacitor. Ferroelectric random access memory is usable without a battery and operable at a high speed. Its development to non-touch cards (radio frequency identification, herein after abbreviated RF-ID) is going to start, and its use in replacement of existing SRAM (static random access amemory), DRAM (dynamic random access memory), flash memory, etc.) and also as memory integrated with logic circuit, is hopefully expected.
A ferroelectric capacitor is typically made by using platinum (Pt) as upper and lower electrodes and a PZT (PbZr1-xTiOx) film as its ferroelectric film. For fabricating ferroelectric random access memory in an LSI process using a silicon substrate, a surface of the silicon substrate having formed transistors and other elements is covered with an insulating film such as silicon oxide film, and a lower Pt electrode, PZT film and upper Pt electrode are made by patterning, to thereby make the ferroelectric capacitor. Normally, a Ti or TixOy film, for example, is interposed as a base layer of the lower Pt electrode to improve its adherence.
It is known that, with this conventional ferroelectric capacitor, a reducing gas of hydrogen, etc. generated in a Si-LSI process invites deterioration of the ferroelectric property, that is, degradation of spontaneous polarization. As a countermeasure against characteristic deterioration of the ferroelectric capacitor due to reduction by hydrogen, there have been proposed some protection techniques for preventing entry of hydrogen, etc. into the capacitor portion. Heretofore, however, there have been no simple and reliable techniques.
Ferroelectric capacitors involve various problems such as deterioration of the property caused by process damage, which must be removed, in addition to characteristic deterioration due to reduction by hydrogen.
For example, Japanese Patent Laid-Open Publication No. H 8-335673 discloses a method for covering a ferroelectric capacitor with a diffusion inhibiting film to prevent direct contact of the ferroelectric capacitor of PZT or other material and a SiO2 insulating film and thereby prevent mutual diffusion of elements between them. The publication indicates that TiO2, ZrO2 and Al2O3, for example, are effective as the diffusion inhibiting film. However, this publication deals with exfoliation of the capacitor ferroelectric film by mutual diffusion, and not the problem of characteristic deterioration of the ferroelectric capacitor by diffusion of hydrogen generated, in the fabricating process.
On the other hand, through recent research, the Inventors have come to realize that the use of the TixOy film as the adhesion layer between the ferroelectric capacitor and the SiO2 film invites some undesirable problems, including deterioration of the ferroelectric property by diffusion of Ti into the PZT film, for example.
It is therefore an object of the invention to provide a semiconductor device having a ferroelectric capacitor with an excellent property and is manufacturing method.
A feature of the invention is that, in semiconductor devices having a ferroelectric capacitor whichi comprises a semiconductor substrate, and a ferroelectric capacitor composed of a lower electrode, ferroelectric film and upper electrode sequentially stacked on the semiconductor substrate via an insulating film, at least one of the upper and lower electrodes forming the, ferroelectric capacitor is covered with a hydrogen barrier film which does not contain titanium.
More specifically, the hydrogen barrier film is formed on the surface of at least one of the position between the lower electrode and the insulating film and the. surface of the upper electrode.
In the present invention, thehydrogen barrier film not containing titanium is preferably a metal oxide film having a hydrogen diffusion constant of 10xe2x88x925 cm2/s or less. The hydrogen barrier film not containing titanium is required to have a high resistance if it is formed so as not to short-circuit the electrodes above on and under the ferroelectric capacitor. In this case, the metal oxide film preferably has a specific resistance not lower than 1 kxcexa9cm. By making such a hydrogen barrier film on at least one of the base layer of the lower electrode and the upper surface of the upper electrode, deterioration in property of the ferroelectric film due to reduction by hydrogen is prevented. Additionally, by selecting an appropriate material for the hydrogen barrier film, it functions as an adhesion layer and exfoliation of the capacitor formed on the insulating film is prevented.
In this specification, the xe2x80x9chydrogen barrier filmxe2x80x9d pertains to a barrier film against diffusion of a reducing gas like fluorine other than hydrogen gas, and it is used because of its function to prevent damage to the ferroelectric film in the fabrication process. Still in this specification, the xe2x80x9cferroelectric capacitorxe2x80x9d pertains to a capacitor not only of a type formed independently from a transistor like in ferroelectric random access memory of a one-transistor/one-capacitor type memory cell structure but also of a type formed in the gate portion of a transistor integrally therewith like in a memory cell of a one-transistor type ferroelectric random access memory.
Still in this specification, the xe2x80x9cmetal oxidexe2x80x9d pertains to a oxide not only of a type including a single metal element but also of a type including more than two metal elements.
In the present invention, when the hydrogen barrier film is interposed under the lower electrode, it is made simultaneously with the lower electrode by patterning. In a structure where the ferroelectric film of the ferroelectric capacitor and the lower electrode are formed to lie over a larger area than that of the upper electrode, the hydrogen barrier film is made to extend from the upper surface of the upper electrode to its side surface and then onto the upper surface of the ferroelectric film. Alternatively, if the lower electrode of the ferroelectric capacitor is made to lie over a larger area than those of the upper electrode and the ferroelectric film, the hydrogen barrier film is made to extend from the upper surface of the upper electrode, covering its side surface and further the side surface of the ferroelectric film, onto the upper surface of the lower electrode.
In the present invention, the ferroelectric capacitor may be configured so that at least a part thereof from the lower electrode to the ferroelectric film is buried in a groove formed in the insulating film. In this case, the hydrogen barrier film is buried in the groove so as to cover the bottom and side surfaces of the ferroelectric capacitor. The hydrogen barrier film may be brought into direct contact with the ferroelectric capacitor, or may be buried not to contact directly. In this case, the hydrogen barrier film is preferably formed also on the upper surface of the ferroelectric capacitor.
The present invention is also characterized in that, in a semiconductor device which comprises a semiconductor substrate, a ferroelectric capacitor having a lower electrode, ferroelectric film and upper electrode sequentially stacked on the semiconductor substrate via an insulating film, and an inter-layer insulating film formed to cover the ferroelectric capacitor, the inter-layer insulating film has a multi-layered structure including first and second inter-layer insulating films, and a hydrogen barrier film is buried between the first and second inter-layer insulating films.
That is, the hydrogen barrier film need not be in direct contact with the ferroelectric capacitor. Also when the hydrogen barrier film is buried inside the inter-layer insulating film covering the ferroelectric capacitor, it is possible to prevent diffusion of hydrogen into the ferroelectric capacitor and prevent deterioration of the ferroelectric property. In this case, thickness of the portion between the hydrogen barrier film and the ferroelectric capacitor is preferably in the range from 0.05 times to three times of the thickness of the ferroelectric capacitor, and this portion functions to prevent reaction which will occur if the hydrogen barrier film and the ferroelectric capacitor contact directly.
Furthermore, in the present invention, if the hydrogen barrier film is interposed between the lower electrode and the underlying insulating film, the hydrogen film is processed by dry etching in self alignment with the ferroelectric film and the lower electrode. In this case, when the etching surface becomes near a vertical surface, etched substance of the hydrogen barrier film again stacks on the side surfaces of the ferroelectric film and the lower electrode already processed, and this forms an effective hydrogen barrier film on side surfaces of the capacitor.
It is preferable that the hydrogen barrier film has a large specific resistance as referred to above. From this viewpoint, the following materials are used, depending on its location.
As the hydrogen barrier film interposed between the lower electrode of the ferroelectric capacitor and the insulating film, one having a small resistance is acceptable,: and it is made of at least one kind of material selected from metal oxides such as Al2O3, AlxOy, AlxSiyOz, AlN, WN, SrRuOy, IrxOy, ZrxOy, RuxOy, SrxOy, RexOy, OsxOy, and MgxOy. For the hydrogen barrier film, these metal oxides can be usable in either an amorphous state, a microcrystalline state, a polycrystalline state or a crystalline state. The same applies to the hydrogen barrier layer formed on the surface of the upper electrode of the ferroelectric capacitor.
The hydrogen barrier film which extends from the upper surface of the upper electrode of the ferroelectric capacitor, through its side surface and the side surface to the ferroelectric film, onto the upper surface of the lower electrode, is required to have a high resistance, more specifically, a specific resistance not lower than 1 kxcexa9cm, and at least one material is selected for it from metal oxides such as Al2O3, AlxOy, AlxSiyOz, ZrxOy and MgxOy.
The hydrogen barrier film buried in the inter-layer insulating film is made of at least one of materials selected from metal oxides such as Al2O3, AlxOy, AlxSiyOz, TixOy, ZrxOy, MgxOy and MgxTiyOz.
The present invention is also characterized in that, in a semiconductor device which comprises a semiconductor substrate, and a ferroelectric capacitor having a lower electrode, ferroelectric film and upper electrode sequentially stacked on the semiconductor substrate via an insulating film, a first SrxRuyOz film is interposed between the ferroelectric film and the lower electrode, a second SrxRuyOz film is interposed between the ferroelectric film and the upper electrode, and these first and second SrxRuyOz films have thicknesses Tsro(BE) (nm) and Tsro(TE) (nm) in the range satisfying 10xe2x89xa6Tsro(BE)+Tsro(TE)xe2x89xa6(2/12)Tpzt with respect to the thickness Tpzt of the ferroelectric film.
By inserting SrxRuyOz films on the upper and lower boundaries of the ferroelectric film while controlling their sum thickness to be in a certain range relative to the thickness of the ferroelectric film, the fatigue characteristic of the ferroelectric capacitor is improved significantly. Particularly, it has been experimentally confirmed that their sum thickness in the range of Tsroxe2x89xa6(2/15)Tpzt is more preferable.
The present invention is also characterized in that, in a semiconductor device which comprises a semiconductor substrate having formed a transistor, an insulating film having a contact plug buried therein for connection to a diffusion layer of the transistor, and a ferroelectric capacitor formed on the insulating film and connected to the transistor via the contact plug, the ferroelectric capacitor includes a lower electrode, ferroelectric film formed on the lower electrode and having the same area as that of the lower electrode, upper electrode formed on the ferroelectric film and having a smaller area than that of the ferroelectric film, and protective film formed on side surfaces of the upper electrode in self alignment therewith to cover the surface of the ferroelectric film.
By thus making the protective film on the side walls of the upper electrode in self alignment therewith in the COP (capacitor on plug) structure forming the ferroelectric capacitor on the contact plug, the structure with the ferroelectric film having fringes can be obtained in a single process of lithography. This fringed structure prevents deterioration of the ferroelectric property in the fabrication process.
The present invention is also characterized in that, in a semiconductor device which comprises a semiconductor substrate, a ferroelectric capacitor including a lower electrode, ferroelectric film and upper electrode sequentially stacked on the semiconductor substrate via an insulating film, and a wiring formed on the ferroelectric capacitor via an inter-layer insulating film for connection to the upper electrode, the wiring is configured to contact the upper electrode with a contact area not less than 50% of the area of the upper electrode.
By thus increasing the contact area with the upper electrode, the ferroelectric property is effectively recovered by recovery annealing in the process of making the contact hole, and an excellent ferroelectric capacitor can be obtained.
In the present invention, the hydrogen barrier film formed on the capacitor can be used also as a stopper in the process for leveling the inter-layer insulating film on the hydrogen barrier film. Additionally, in a film structure stacking SixNy or SixOyNz on the hydrogen barrier film, better stopping function is expected during CMP (chemical mechanical polishing).
As reviewed above, according to the invention, it is possible to obtain a semiconductor device having a ferroelectric capacitor excellent property by preventing or minimizing characteristic deterioration of the ferroelectric capacitor due to reduction by hydrogen in the fabricating process.